Optical scanning devices are conventionally used to form images in laser beam printers and similar devices. An optical scanning device emits a light beam, conventionally a laser beam, that scans as a light spot along a scanned surface where photosensitive material is present. More precisely, the optical scanning device includes a collimator lens to collimate a light beam emitted from a light source, such as a semiconductor laser device, and then uses an optical deflector such as a high-speed rotating polygon mirror in order to deflect the collimated light beam onto a scanned surface, such as a photosensitive drum surface.
Multi-beam scanners that simultaneously use plural light beams to scan are in development for laser printers, including color laser printers. A multi-beam scanner uses an optical system to guide plural light beams emitted from a light source to a shared polygon mirror. The polygon mirror is rotated to reflect the plural light beams to different points on scanned surfaces. In order to guide the plural light beams to multiple scanned surfaces from a shared polygon mirror, the plural light beams have to be separated from one another. Therefore, a separation optical system such as a splitting mirror is provided between the polygon mirror and the scanned surfaces.
Recent demand for higher printing speed requires higher scanning speed, which in turn requires higher rotation rates of the polygon mirror. To achieve higher rotation rates, the polygon mirror must be made smaller and lighter. For that to occur, it is important that the polygon mirror have a small thickness in the sub-scanning direction that is generally parallel to the axis of rotation of the polygon mirror and that is orthogonal to the main scanning direction in which rotation of the polygon mirror causes the laser beams to scan.
The multi-beam scanner includes plural light beams arranged in the sub-scanning direction. Therefore, the polygon mirror has a larger thickness in the sub-scanning direction than that of a single beam scanner. Consequently, the polygon mirror is heavier and less compact. It is understood that the plurality of light beams may be spaced closer together in order to reduce the thickness of the polygon mirror in the sub-scanning direction. However, a limitation is imposed on this close spacing by the fact that the plurality of light beams must be separable on the image side of the polygon mirror. Thus, it is difficult in conventional multi-beam scanners to make the polygon mirror thin in the sub-scanning direction as required to increase printing speed.